Development of small information equipment has rapidly advanced miniaturization of its electronic components mounted thereon in recent years. Ball grid array (hereinafter, referred to as “BGA”) in which electrodes are arranged on a rear surface of the electronic component has been applied to the electronic components in order to cope with narrowing of the terminals and/or reduced size of mounting area according to downsizing requirement.
As the electronic component to which BGA is applied, a semiconductor package is exemplified. In the semiconductor package, a semiconductor chip having electrodes is sealed by any resins. On the electrodes of the semiconductor chip, solder bumps are formed. Each solder bump is formed by connecting a solder ball with the electrode of the semiconductor chip. The semiconductor package to which BGA is applied is mounted on a printed circuit board by connecting the solder bump fused by heating with conductive land of the printed circuit board. Further, in order to cope with any higher density mounting requirement, a three dimensional high density mounting structure in which the semiconductor packages are piled along a height direction thereof has been studied.
However, when BGA is applied to the semiconductor packages on which the three dimensional high density mounting is performed, each solder ball may become flat by weight of the semiconductor packages. If such a case occurs, the solder sticks out of the electrodes, so that the electrodes may be connected to each other, thereby causing a short-circuit therebetween.
Accordingly, a solder bump including a Cu core ball which is connected to an electrode of an electronic component has been studied. The Cu core ball is referred to as a ball in which solder film is formed on a surface of the Cu ball. The solder bump formed using the Cu core ball can support the semiconductor package by the Cu ball which does not melt at a melting point of the solder when mounting the electronic component on the printed circuit board even if the weight of the semiconductor packages is applied to the solder bump. Therefore, it is impossible for the solder bump to become flat by the weight of the semiconductor packages. As a related art, patent document 1 is exemplified.
The patent document 1 discloses an invention of the Cu ball having high sphericity and describes the Cu core ball in which the solder film is formed on the Cu ball. The same document also discloses a Pb—Sn solder, components of which are Pb and Sn, as an example. The same document further discloses methods of forming the film such as a plating method, a fusing method, a brazing method and the like, in which they are methods as equivalent methods. Among them, as the plating method, an electrolytic plating method such as barrel plating is disclosed.
By the way, the miniaturization of electronic components has allowed the high density mounting structure but such high density mounting structure has caused any problem of software errors. These software errors may occur by rewriting contents stored in a memory cell of a semiconductor integrated circuit (hereinafter, referred to as “IC”) when entering a ray into the memory cell. It is considered that the α ray radiates by a decay of a radioactive isotope such as U, Th and 210Po in a solder alloy. Accordingly, a solder material decreasing radioactive isotope content and having less α ray has been developed in recent years. As a related document, for example, patent document 2 is exemplified.
The patent document 2 discloses an invention of a Sn ingot having low α dose and discloses that in order to decrease the α dose, electrolytic refining is not only performed but an adsorbent is also suspended into an electrolyte, so that it adsorbs Pb and/or Bi to decrease the α dose.
Patent Document 1: International Publication No. 95/24113
Patent Document 2: Japanese Patent No. 4472752
A problem of this invention is to provide a Cu core ball that prevents any software errors from occurring and specifically, to provide a Cu core ball that has less α dose. In the patent document 1, however, any problem to decrease the α dose of the Cu core ball has been not considered at all. The same document also discloses only Pb—Sn alloy as the description of the background art about the solder alloy constituting the solder film. The α ray radiates from 210Po while 210Pb, which is an isotope of Pb contained in Sn as impurities, is decayed from 210Po to 206Pb through 210Bi and 210Po. The Pb—Sn solder alloy which is only one alloy disclosed in the same document contains a lot of Pb so that it is considered that 210Pb which is a radioactive isotope is also contained. Therefore, even if this solder alloy is applied to the solder film of the Cu core ball, it is quite impossible to consider that the Cu core ball disclosed in the patent document 1 indicates low α dose because any problem to decrease α dose is not taken at all into consideration in the same document.
As described above, the patent document 2 discloses that Pb and/or Bi in the Sn ingot are adsorbed to decrease the α dose by removing Pb and Bi from the Sn ingot by the electrolytic refining which is performed under a situation where electrolyte and electrodes are static. The same document, however, does not absolutely disclose that Sn plating is performed on a Cu ball nor disclose that the electrolytic plating is performed under a situation where the Cu ball and electrolyte are dynamic. Further, in the electrolytic refining described in the same document, since electrolytic deposition is limited to a surface along one direction, it is impossible to form a plating film having a uniform film thickness on a small work such as the Cu ball.
Additionally, by the patent document 2, it is impossible to decrease the α dose by only performing the electrolytic deposition of Sn to a plate electrode under a common electrolytic refining because standard electrode potentials of and Bi are near that of Sn. If the electrolytic refining described in the patent document 2 is applied to a formation of plating film on the Cu ball and an adsorbent is suspended into a plating solution to perform a barrel plating, the plating solution and the work are stirred and at the same time, the adsorbent is also stirred. In this case, Pb ions or Bi ions which the adsorbent adsorbs become carrier so that they can be brought into the solder film together with the adsorbent. The solder film into which the adsorbent is brought radiates high α ray. A grain size of the adsorbent is sub-micron order and since it is very small, it is conceivably impossible to separate and collect the adsorbent after the suspension thereof with the plating solution being flown. Therefore, it is difficult to avoid bringing the adsorbent to which Pb and/or Bi is (are) adsorbed into the film.
Moreover, the patent document 1 discloses the Pb—Sn solder alloy but the plating method, the fusing method, the brazing method and the like are disclosed as equivalent methods so that it rather describes to deny decreasing the α dose. The problem of the patent document 1 is to manufacture a Cu core ball having a high sphericity. On the other hand, in order to solve the problem to decrease the α dose, the patent document 2 discloses that Pb is removed from Sn as much as possible by the electrolytic refining. Therefore, a person who has an ordinary skill in the art and knows the patent document 1 does not conceive the problem described in the patent document 2 to decrease the α dose of the Cu core ball. Their solder components are also contrary to each other so that it is conceivable that he requires a process of trial and error over and over again to conceive the problem to decrease the α dose and to conceive the application of Sn series solder among an infinite number of solder alloys instead of Pb—Sn solder alloy constituting the solder film.
Thus, the person who has an ordinary skill in the art could not combine the patent document 1 with the patent document 2. Further, it is very difficult for the person who has an ordinary skill in the art to prepare the plating solution using the Sn ingot having low α dose, which is disclosed in the patent document 2, and to form the Cu core ball by the plating method disclosed in the patent document 1.
Accordingly, when the Cu core ball manufactured by applying the conventional technologies described in the patent documents 1 and 2 is used for forming the joint, there is such a high possibility that radioactive elements presented in the solder film of the Cu core ball are spread over the electrode of the joint and the α ray radiates therefrom. Therefore, even if the conventional technologies are combined, it is impossible to decrease the α dose of the Cu core ball so that it is impossible to avoid any soft errors, which have been a problem newly generated by the higher density mounting requirement.